Finally we also mention that there is a huge literature on the study of higher-order elliptic operators \(P(D)+V(x)\)  in many topics in mathematics and mathematical physics. In addition to the aforementioned works [1]}, [2]}, we refer the readers to e.g. [3]}, [4]}, [5]} for the spectral and scattering theory, [6]}, [7]} for Harmonic analysis and [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]} for various dispersive properties such as time decay, local energy decay, Strichartz estimates for \(e^{-itH}\) , and the asymptotic expansion and uniform resolvent estimates for \((H-z)^{-1}\) .
